An orbiting satellite is in continuous need of data relating to its position in order to maintain or change its orientation with respect to earth and its orbital path. In addition the adjustment of antenna arrays, solar arrays, and other common functions is also dependent on this information. Much of these data is currently uploaded from ground stations either automatically or by manual intervention. This cannot be accomplished on a continuous basis because of gaps in communication as the satellite orbits the earth.
Attitude control is a primary example of a function which requires such information. Modern satellites now have an extensive means to accumulate information by means of onboard sensing of the positions of the earth, sun, stars, and other satellites. In addition an orbital calendar (ephemeris) may be stored in onboard computers which contain the expected orbital path and schedule of attitudes. A further database of expected repetitive perturbations can be stored and updated by actual experience. From this data an orbital model can be predicted and used to correct errors in attitude, orbital position and the focus of the various arrays.
Where a geosyncronous orbit is involved, the data is relatively stable, since the relative position of earth remains the same. The data becomes more dynamic as the orbit becomes more complex. With the advent of commercial satellites, there is a greater demand for complex orbits which provide greater "hang time" over specified areas of high demand. An example of such an orbit is a so called Tundra orbit which is an inclined elliptical sidereal day orbit. The relative position of the earth in such orbits is no longer constant and there is a continuous need to update data with respect to earth and other referenced bodies in the galaxy. It is anticipated that, when such orbits are used, the orbital and attitude data will need to be updated as frequently as every ten seconds, depending on the application served by the satellite.
There is a need, therefore, for a method of propagating the required information which is resident onboard. The information needs to be translated into a reference frame suitable for use by the various functions. It is a purpose of this invention to provide an onboard system for generating near real time knowledge of the spacecraft position and velocity to more efficiently perform station keeping and more accurately control attitude, solar arrays, and antenna pointing based on data sensed onboard by earth, sun and other sensors.